The present invention relates to a novel styrene-anhydride copolymer containing amido group. The present invention further relates to a process for producing the copolymer and to the use of the copolymer as a photo resist in an alkaline soluble development of an irradiative reaction and as a photo resist in printed circuit boards.
In the printed circuit board field, it is well known to use an excellent photosensitive resin composition as a photo resist for mask solder and chemical electroplating. The purpose of using the mask solder is to control the area of solder, to avoid the interconnection between solders, to prevent a copper conductor from being exposed and corroded, and to maintain the insulation between conductors for a long period. It is generally to use the thermosetting resins such as an epoxy resin or aminoplast resin as a main component of the printed solder.
Recently, however, as the demand for the high-density of printed circuit board and the insulation between conductors increase, the solder mask is required to have an excellent precision in film thickness and an excellent precision in size. A screen-printing is not satisfactory for such demands.
Therefore it is desired to develop a solder mask having excellent size precision and reliability produced through the formation of a thick film on a conductor (preferable a thickness of 25 micrometers) by a photolithography which is a process for producing a pattern through continuously exposing the pattern and developing it.
Conventionally, as a photosensitive resin for forming a solder mask, it is common to employ a photosensitive resin comprising acrylic polymers and photopolymerizable monomer as main components, such as disclosed in Japanese Patent Unexamined Publication Nos. Sho 53-56018 and Sho 54-1018.
However, in order to impart the photosensitive resin to a film-formation property, it is necessary to use a great amount of acrylic polymers so that the heat-resistance of a resultant cured film is insufficient.
Furthermore, Japanese Patent Unexamined Publication No. Sho 54-82073 discloses a composition comprising a photosensitive epoxy resin containing a cinnamophenone (chalcone) in the main chain and an epoxy resin-curing agent. The composition is an excellent heat-resistant composition. However, the sensitivity of the photosensitive composition is insufficient to form a thick film photo resist. Additionally, since the composition needs to use inflammable solvents such as cyclohexanone as a developing solution, the safety becomes an important issue in the manufacturing process. Moreover, Japanese Patent Unexamined Publication No. Sho 61-272 discloses a composition comprising an epoxy-containing phenolic novolac resin type epoxy acrylate and a photopolymerization initiator as main components. The composition could form a thick film solder mask after curing. Although the composition has an excellent heat-resistance, it needs to use a mixture of trifluoroethane and lower alcohols as a developing solvent, so it also has a safety issue.
Furthermore, Japanese Patent Unexamined Publication No. Hei 2-160242 discloses a photosensitive resin composition comprising an epoxy acrylate resin containing carboxyl group. Japanese Patent Unexamined Publication No. Hei 2-166452 discloses an alkali soluble photosensitive resin composition comprising a carboxyl group-containing styrenic polymer. If an imaging pigment such as colorless crystal violet, which color will change when exposing to ultraviolet light, is added to both resin compositions disclosed in the above two prior references, the color of the imaging pigment will change from colorless to purple blue due to its poor stability and the pigment will loss its ability to change color upon exposing to ultraviolet light. It causes the trouble to operators since they cannot judge whether the exposing process is completed. Hence, what the operators can do is to repeat the exposing process. Such instability will not only make the process of production complicated but also cause low throughput, high cost and increasing amount of waste. Therefore, it generally uses the resin composition without incorporating the imaging pigment (color changeable pigment).